I recently upgraded to 11.2 and found that my styles to add a checkbox to the record selector no longer handles null values.  When there is a mixture of true and false values then the checkbox in the header should be a square, and that used to work in previous versions.  In fact, the first picture in Josh Smith's blog shows it correctly.

http://blogs.infragistics.com/blogs/josh_smith/archive/2008/09/04/adding-checkboxes-to-the-record-selectors-in-xamdatagrid.aspx

The problem is easily visible in the feature browser on the "Checkbox in Record Selectors" page.  see screenshot.